1. Field of the Invention
The present invention generally relates to image forming devices, and particularly relates to an image forming device which prints out information on a recording sheet by making a latent image on a recording drum.
In recent years, electronic photographic printers of a serial type have been developed, which type carries an electronic photographic process unit on a carriage for printing information. This development was made in order to meet a demand for miniaturization and lower pricing of electronic photographic recording devices. These printers of the serial-type duplicate an image on to a recording sheet with an aid of an image-transfer unit by running the carriage in a direction perpendicular to a direction in which the recording sheet is led. The image transferred on to the recording sheet is then fixed by an image-fixing unit having a shape of a roller. However, a recent development of such printers still needs further refinement and improvement in printing qualities.
2. Description of the Prior Art
FIGS. 1A and 1B show a plan view of part of a serial-type electronic photographic printer of the prior art and a cross-sectional view of a carriage of FIG. 1A, respectively.
In FIGS. 1A and 1B, a serial-type electronic photographic printer 11 disclosed in Japanese Laid-Open Patent Application No.61-152463 includes conveying rollers 13a and 13b, which convey a recording sheet 12. The serial-type electronic photographic printer 11 also includes a shaft 14 parallel to axes of the conveying rollers 13a and 13b, and a carriage 15, which is movable in a direction along the width of the recording sheet 12 (perpendicular to the direction in which the recording sheet 12 is conveyed). The movement of the carriage 15 is guided by the shaft 14 and driven by a motor (not shown). The serial-type electronic photographic printer 11 also includes an image-fixing unit 16 fixed in a predetermined position, which has a width wider than that of the recording sheet 12. There is an image-transfer unit 17 provided beneath the recording sheet 12, as shown in FIG. 1B.
The carriage 15 includes an image-holding body 21, an electrification unit 22, a light-exposure unit (an LED array and a lens) 23, a developing unit 24, toner 25, and a developing roller 26. The image-holding body 21 rotates at a speed corresponding to the movement of the carriage 15. A surface of the image-holding body 21 is electrified by the electrification unit 22, and the light-exposure unit 23 forms a latent image of static charge on that surface. The latent image of static charge is turned into a visible toner image by the developing roller 26 of the developing unit 24 attaching the toner 25 on the above-mentioned surface. The toner image on the surface of the image-holding body 21 is transferred on to the recording sheet 12 by the image-transfer unit 17 opposing the image-holding body 21 through the recording sheet 12. The image transferred on to the recording sheet 12 is fixed by the image-fixing unit 16.
FIG. 2 shows another example of a serial-type electronic photographic printer of the prior art. The figure shows an inside structure of the carriage 15, which is disclosed in the Japanese Utility Model Laid-Open Application No.61-145649. The carriage 15 is provided with an image-fixing unit 27 in the inside thereof. The carriage 15 includes a cleaner 30, which is used for rubbing off remaining toner from the image-holding body 21 after finishing printing.
The image-fixing unit 27 is provided with an image-fixing lamp 28, and rotates in the same direction as does the image-holding body. Inside the image-fixing roller 28, a heat source 29 such as a halogen lamp is provided as heating means. The image-fixing roller 28 is heated by the heat source 29 up to a predetermined temperature prior to a printing operation. The temperature of the image-fixing unit 28 is monitored by a temperature detecting unit such as a thermistor, and is controlled during the printing operation. The image-fixing unit 27 is moved along with the image-holding body 21, and fixes an image right after the image is transferred.
Here, the transfer of the image by the image-transfer unit 17 is carried out by applying a predetermined voltage level between the image-holding body 21 and the image-transfer unit 17. Thus, the image-transfer unit 17 is formed from a plate such as a conductive rubber plate.
Although not shown in the figures, there is another type of a serial-type electronic photographic printer, in which an image-holding body (photosensitive drum) can be retracted from a surface of the recording sheet during a time of conveying the recording sheet. The retraction of the image-holding body is carried out by using a guide rod of the carriage as a fulcrum. The carriage also includes an image-fixing unit having a heat source, which illuminates light upon the recording sheet to effect a flash fixing of an image. An example of such a heat source is a halogen lamp and a xenon lamp for emitting an infrared ray. This serial-type electronic photographic printer is disclosed in the Japanese Patent Laid-Open Application No.56-77167.
The serial-type electronic photographic printer shown in FIG. 1 has a couple of problems associated with its design. In this printer, the recording sheet 12 is not conveyed continuously, but the movement of the recording sheet 12 is sporadic. Thus, a time length for which the image-fixing roller touches the recording sheet varies widely from portion to portion of the recording sheet bearing a toner image. That is, when the recording sheet stays still, a portion thereof touching the fixing roller ends up being in contact with the fixing roller for a long period. On the other hand, when the recording sheet is moved, a portion passing under the fixing roller does not have as long a contact with the fixing roller. This results in a variation in a degree of the fixing of the image, leading to a degraded image quality. Also, the contact of the image-holding body 21 with the recording sheet 12 lifts the recording sheet 12, creating a paper jam at the beginning and at the end of the recording sheet.
The serial-type electronic photographic printer shown in FIG. 2 also has a problem. In this printer, the image-holding body 21 and the fixing roller 28 are always in contact with either the image-transfer unit 17 or the recording sheet 12. Generally, the image-fixing roller 28 has a width of 30 mm, and is urged with pressure ranging from 1 kg to 2 kg. The recording sheet 12 on which the pressure is locally applied tends to be lifted from the image-transfer unit 17. The recording sheet 12 is also lifted by the image-holding body 21. These two forces to lift up the recording sheet 12 create a culminating effect which causes mismatch between a rotation rate of the image-holding body 21 and the speed of the movement of the image-holding body 21 on the recording sheet 12.
FIG. 3 shows this culminating effect observed in the serial-type printer of the prior art. As shown in FIG. 3, pressure by the image-holding body 21 and the image-fixing roller 28 lifts the recording sheet 12, resulting in the mismatch between the rotation rate and the speed of the movement. This results in the lifting of the recording sheet between the image-holding body 21 and the image-fixing roller 28, leading to a degraded image quality and jamming of sheets.
Also, when the image-fixing unit employs a heat source fixing an image by light illumination, sufficient heat cannot be applied to the recording sheet. This also creates a problem that the fixing of the image is not sufficient.
When an infrared lamp is used, an infrared ray emitted by the lamp has too low an energy density to fix an image within a short time. Also, when the recording sheet is jammed, there is a high probability of the recording sheet catching on fire. When a xenon lamp is used for a flash fix, a condenser of a large capacity become necessary. Also, the flash fix causes problems of flying toner and annoying noise. Furthermore, it is difficult to have a continuous illumination of light for the serial-type printer. Both the infrared lamp and the xenon lamp are bound to be placed near the photosensitive drum, so that the sealing of the drum from the light exposure is difficult to attain. Also, since only the image-holding body touches the recording sheet without the image-fixing unit touching it, the recording sheet may be lifted up to cause deficiencies in a printing quality.
Accordingly, there is a need in the field of serial-type electronic photographic printers for a serial-type electronic photographic printer which can prevent the lifting of a recording sheet and improve the printing quality.